1. Field of the Invention
The present invention relates to machinery and processes for manufacturing spaced apart thermal seals in applications such as pouch-type bags in which metered quantities of substances, such as food stuffs, are sealed therein.
2. Description of the Prior Art
The Assignee of the present invention manufactures machinery for making thermally sealed pouch-type bags. Machines of this type are well known. A continuous web having the front and back sides of the bag printed thereon is folded in half and run over the outside periphery of a first sealing system to which the present invention relates in part. The sealing system has evenly spaced thermally actuated sealing elements mounted on the periphery of a rotating wheel which forms parallel spaced apart seals by melting a thermoplastic material coated on the inside of the web after the web has been folded in half. After the parallel seals are made by the sealing system, each bag in the web has been formed except for the closure of the top. After running across the sealing system, the continuous web with parallel spaced apart seals which define the opening of each bag is fed to a pouch filling station at which a metered quantity of substance, such as powered food, is placed inside the individual bags located along the continuous web as the bags at the pouch filling station rotate around the periphery thereof. After filling with the metered quantity of substance, the bags then pass to a second thermal sealing station which heat seals the top of each bag to close the food stuff or other material therein in a sanitary manner. Finally, the sealed bags, which are still part of the continuous web, pass to a cutting station at which a series of rotating knives cut the individual bags apart uniformly in the middle of the parallel spaced apart thermal seals which were formed by the first sealing system.
The Assignee of the present invention manufactures machines of the aforementioned type which operate at high processing speeds, such as up to 4000 linear inches per minute or higher, to make individual pouch-type bags from the moving web. High speed operation in machinery of this type is extremely important as a consequence of the economics of manufacturing products such as dry packaged foodstuffs. Any circumstance occurring during manufacturing of filled thermally sealed pouch-type bags which requires the line to be shut down has a serious effect on the productivity of the manufacturing operation.
Materials from which thermally sealed pouches are manufactured include paper, cellophane foil, metallized structures, polypropylene and other laminates of preprinted or unprinted stock.
In the prior art, two basic types of sealing systems, which typically operate with the axis of rotation of the sealing wheel of the sealing system being vertical, are known. These sealing systems are known as xe2x80x9cdrumxe2x80x9d and xe2x80x9cmodularxe2x80x9d sealing systems.
The drum style sealing system is a single piece of metal machined from a casting which has a calculated land diameter that is the heated surface contacting the moving web to form the spaced apart parallel seals. Drum style sealing systems have the disadvantage of not having an adjustable spacing between the parallel seals which define the width of the pouch-type bag. Behind each land and parallel thereto is a hole containing a tubular electric heater element. Drum style sealers work well once the substantial thermal mass of the machined casting has come up to operating temperature which is typically a temperature of 280xc2x0 F. or above. The drum, as a consequence of its high thermal mass, will produce unacceptable seals if the continuously moving web has to be stopped for any reason. This high thermal energy of the drum at the point of contact of the lands with the web for a substantial period of time either causes the parallel seals to be destroyed because of the prolonged contact or the parallel seal grow to a greater width than desired which restricts the size of the bag. A reduced volume of the bag caused by wider seals either prevents the desired metered quantity of material from being placed inside the bag or worse, the spacing between the bags at the cutting station becomes out of synchronism with the spacing of the cutting knives causing severing of the web at spaced apart locations which do not correlate with the middle of the thermally formed seals which can cause the bags to have faulty seals or spill the contents thereof.
The modular sealing system is illustrated in FIG. 1. The modular style sealing system 10 includes a wheel having a stationary lower circular plate 12 and an upper rotating annular ring 14 to which individual seal forming elements 16 are mounted by threaded type connectors 17. The seal forming elements make peripheral contact with a continuous web 18 as illustrated in FIG. 2. The wheel of the sealing system 10 is driven by rotating shaft 18. Cooling air is blown from holes 19. The individual seal forming elements 16 may be moved radially in slots 20 to permit adjustment of the spacing between the parallel seals 24 by loosening and tightening of the connectors 17. Each of the sealing elements 16, like the aforementioned drum style sealer wheel, has a tubular electric heating element (not illustrated) mounted in a longitudinally extending hole in each seal forming element 16.
The modular style sealing system 10 is a substantial improvement over the drum style sealing system by permitting adjustability of the spacing between individual spaced apart seals 24 on the continuously moving web 18. The overall thermal mass of the sealing elements 16 results in a substantial variation in the temperature along the longitudinal dimension thereof. The modular style sealing system typically operates with a temperature differential along the longitudinal dimension between the top and bottom of the individual sealing elements 16 of 30xc2x0 to 40xc2x0 F. and between 30xc2x0 to 50xc2x0 F. between different sealing elements 16. In part, the difference in temperature along the longitudinal dimension of the sealing elements 16 during operation is caused by heat flow from the electrical heating element of the individual sealing elements 16 to the top rotating annular plate 14 during rotation and stoppage thereafter. In order to obtain a high quality seal across the length of a pouch-type bag, it is necessary to operate the modular sealing wheel at a temperature which, at its lowest point is a temperature such as 280xc2x0 F., which can result in some of the individual sealing elements 16 operating at the center thereof, which are more thermally insulated than the top and bottom portions of the sealing wheel, at a temperature of at least 320xc2x0 F.
As illustrated in FIG. 2, during normal operation, the continuous web 18 rolls across the periphery of the individual sealing elements 16. The sealing elements 16 have precisely chosen spacings therebetween so that each peripherally outwardly facing surface 22 contacts the continuous web 18 at the same place as the web moves around the periphery of the wheel. The result of the precise registration during moving of the continuous web 18 in contact with the outer surfaces 22 of each sealing element 16 is the formation of a straight uniform width seal 24 even though there is the aforementioned temperature variation of 30xc2x0 to 40xc2x0 F. during operation between the center of the individual heating elements and the ends thereof. Because of the high speed rotation of the web across the sealing wheel 10, there is a controlled short dwell time which produces uniformity in seal width, even though the aforementioned temperature variation exists.
However, if, for any reason the continuous movement of the web 18 has to be stopped, the higher operating temperature of the midpoint of the sealing elements 16, which have substantial thermal mass, causes the individual surfaces 22 to contact the continuous web 18 sufficiently long to produce a seal 26 which grows in width toward the midpoint. The wider seal 26, produced as a result of the longer contact time of the exterior surfaces 22 with the web 18 during stoppage, produces individual bags having an hour glass type interior shape which reduces the capacity of the bag to be filled. After the machine is again started, seals of the shape 26, when they pass to the filling station of the pouch-type bags, often prevent the precisely metered quantity of material, which is normally placed in the bags during continuous operation, from being placed in the individual pouch-type bags. As a result, the volume of material sealed in the bags is reduced and may vary. This can cause spillage of material because it is left over from the precisely metered quantity which is made available to be placed into the pouch-type bag and tends to fall from the elevated filling station. This is wasteful and further can require costly cleanup operations. Finally, the thermally sealed bags, as a result of having less material therein than the desired amount, may not register precisely with the downstream cutting knives at the cutting station which can result in the continuous web being severed at points which are not at the midpoint of the individual seals 26, resulting in total loss of the material.
Pouches having the hourglass interior, as illustrated in FIG. 2, are much more likely to cause jams in the feeding of the web 18, may cause stoppage of the processing and even breakage of the web. Rethreading of the web is a time consuming and costly process when stoppage of the manufacturing of individual pouches is taken into consideration. Finally, even if there is no appreciable spillage at the station which fills the bags and if the bags are properly severed, downstream quality control should reject the individual bags as not containing the correct weight of material.
The modular style sealer systems also have a large thermal mass which requires approximately 8-10 minutes to heat the sealer to the set point temperature at which thermal sealing is performed. This heat up time can result in a loss of 8,000-10,000 pouches given that machinery of this type is designed to operate at speeds of 1,000 pouches or higher per minute. Furthermore, when stoppage occurs, as illustrated in FIG. 2, it takes several minutes for the sealing system to cool down so that the web 18 will not be burned or produce the widened seals 26 which, in the past, in some applications, have required the blowing of cooling air from holes 19 in the lower circular plate 12 and is a significant cost to the manufacturer of pouch-type bags.
The present invention is an improved sealing system and process of operation thereof which eliminates the problems of the prior art. Sealing elements in accordance with the invention have faster heat up and cooling down times and consume less thermal energy than the sealing system of FIG. 2. Each seal forming element has a section on which at least one electrical heater is mounted, a mounting section at which the sealing elements are attached to upper and lower parts of the wheel or other mounting structure in non-rotary applications and a section including thermal insulation which insulates the section on which the at least one heater is mounted from the mounting section. The thermal insulation, which in a preferred application is an open slot, prevents substantial heat conduction to the mounting section, such as to the upper and lower parts of the sealing wheel, resulting in only the section on which the electrical heater is operated being operated at the high temperatures required to form thermal seals. As a result, a very quick heat up time of less than one minute is achieved to reach thermal sealing temperatures and furthermore, the cool down time, after a stoppage, is also substantially lessened. The formation of the seals 26 and operational difficulties of the prior art are eliminated.
A preferred embodiment of the invention utilizes thick film electrical resistances of known design preferably distributing the electrical resistance in sections of different resistance. The electrical resistance sections adjacent to the end parts of the outer section of the sealing element through which heat flows through an intermediate section to an inner mounting section may have a higher resistance and other sections of the electrical resistance, which are disposed adjacent to the thermal insulation, may have a lower electrical resistance. The overall distribution of the electrical resistance in sections or otherwise may be varied to suit any distribution of electrical resistance which compensates for heat loss through the mounting structure of the individual sealing elements to the wheel and/or processing conditions and applications to facilitate the making of different types of seals on different types of materials of different size and strength for different applications. For example, a rotary embodiment of the present invention, as described below, achieves only a 14xc2x0 F. difference along the entire longitudinal length of the sealing element, while having the enhanced heat-up and cool-down lines and lower energy consumption. The electrical resistance may be distributed along the longitudinal dimension of a substrate on which the electrical resistance is located as a series of sections of electrical resistance having different resistance values as described above or in a smooth resistance gradient which changes progressively along the longitudinal dimension of the substrate on which the electrical resistance is located toward the midpoint or otherwise to suit diverse sealing applications.
In one preferred embodiment of the invention in which an inner mounting section of each seal forming element is joined to upper and lower parts of a sealing wheel which are separated on an axis of rotation of the wheel, the higher electrical resistance is located proximate to the upper and lower parts of the sealing wheel and the lower electrical resistance separates the higher electrical resistance proximate to the upper and lower parts. The higher electrical resistance proximate to the upper and lower parts of the sealing wheel compensates for the increased heat flow thereto as a consequence of heat flow-through two projections of the intermediate section of each sealing element, between which an open slot is disposed, separating the section on which the electrical resistance is located and the inner mounting section at the longitudinal ends of the sealing element. Preferably, the insulation contained in the intermediate section is the open slot which separates the two projections but the invention is not limited thereof.
The present invention has substantial advantages over the prior art. First, consistent and easy to change temperature profiles on the heating surface of each seal forming element and between individual sealing elements are achieved consequent from the use of heating elements made from film resistances which preferably are thick film resistances. Faster heat up of the seal forming elements is achieved at the start up of processing. Faster cool down to a set point temperature at stopping of the processing and faster heat up for restarting processing thereafter is achieved. The seal forming elements are readily washed down with cleaning solutions in view of the exterior mounting of the film resistances on the seal forming elements. The overall energy consumption of the sealing system is substantially lessened in view of the much smaller thermal mass of the section on which the film resistance heaters is located, which is the only part of the seal forming element which is substantially heated to produce the thermal seals, with the inner or mounting section of each seal forming element operating at temperatures low enough that they may be touched by an operator without being burned. The lower energy consumption lengthens the life of electrical controls mounted on the sealing system. Also, a lower profile is achieved which facilitates rethreading of the endless web. Additionally, each seal forming element is radially adjustable to vary the spacing between the spaced apart thermal seals.
In one preferred embodiment, a pair of film electrical resistances, which preferably are thick film resistances, are disposed on opposed surfaces of each seal forming element and a seal forming surface which contacts at least one moving web, is opposed to the thermal insulation and intersects the opposed surfaces.
In another preferred embodiment, a single film resistance, which preferably is a thick film resistance, is located on the seal forming surface of each seal forming element which contacts the at least one web on which the thermal seals are formed. An intermediate section of each seal forming element, which is a strip of insulation or an open slot, thermally isolates an inner section of each seal forming element which is connected to the sealing wheel from an outer section.
Additional embodiments of seal forming elements of the present invention have applications as linear non-rotating sealing elements. In these applications the film resistance, which preferably is a thick film resistance, is thermally isolated by insulation from a support. The thermal insulation between the film resistance and the support provides fast heat up and cooling down times and low energy consumption.
During operation of the sealing system in continuous processing of at least one web into pouch-type bags, temperature variation along the electrical heater may be less than 30xc2x0 F., preferably is less than 15xc2x0 F. and most desirably is less than 5xc2x0 F.
Rotation of the sealing wheel to form thermal seals and movement of the at least one web may be stopped with the invention. While the at least one web contacts the periphery of the sealing elements with the electrical potential being turned off to stop heating, seals are achieved with the invention which are perpendicular to a longitudinal dimension of the at least one web and are straight and parallel which eliminates the problems of the thermal sealing systems of FIGS. 1 and 2. As a result, the filling of the pouch-type bags at a downstream filling station, does not have substantial spillage because the capacity of the pouch-type bags has not been reduced as with the prior art of FIG. 2. Furthermore, at the time of cutting the continuous web into pouch-type bags, there is no longer a problem of the bags being out of registration with the thermal seals resulting in spillage of materials in the bags. Stopping of the pouch-type bag forming system including the sealing system may be accomplished without having to rethread the at least one web. Improperly severed bags caused by each bag having a reduced amount of packaged material and the resultant loss of material does not occur. All of these benefits result in a higher throughput of individual bags.
A sealing system which forms spaced apart thermal seals along a moving at least one web by contact of the moving at least one web with heated portions of sealing elements mounted on a sealing wheel during rotation in accordance with the invention includes seal forming elements which peripherally contact the at least one web to form the thermal seals by heat transfer to the at least one web; and wherein each seal forming element has an outer section on which is located at least one electrical heater, an inner section which contacts the sealing wheel and an intermediate section which includes thermal insulation which insulates the outer section from the inner section. Each electrical heater may be comprised of an electrical resistance disposed along a longitudinal dimension of the outer section of the seal forming element, the electrical resistance being distributed along the longitudinal dimension with a higher electric resistance being adjacent to a part of the intermediate section through which heat flows to the inner section and a lower electrical resistance which is disposed adjacent to the thermal insulation of the intermediate section. The inner section of each seal forming element may be joined to upper and lower parts of the sealing wheel which are separated on an axis of rotation of the wheel; and the higher electrical resistance may be located proximate to the upper and lower parts of the wheel and the lower electrical resistance may separate the higher electrical resistance proximate to the upper and lower parts. Each seal forming element is radially adjustable to vary the spacing between the spaced apart thermal seals. A pair of film electrical resistances may be disposed on opposed surfaces of each outer section and a seal forming surface may touch the moving at least one web, may be opposed to the thermal insulation and may intersect the opposed surfaces of the outer section. Each electrical heater may be a film resistance which preferably is a thick film resistance. The outer section may be parallel to an axis of rotation of the sealing wheel and the electrical resistance may extend longitudinally along the outer section. A single film resistance, which preferably is a thick film resistance, may be located in the seal forming surface which contacts the moving at least one web.
A process for forming spaced apart thermal seals along a moving at least one web by contact of the moving at least one web with heated portions of sealing elements mounted on a sealing wheel during rotation having seal forming elements which peripherally contact the at least one web to form the thermal seals by heat transfer to the at least one web and wherein each seal forming element has an outer section at which is located at least one electrical heater, an inner section which contacts the sealing wheel and an intermediate section which includes thermal insulation which insulates the outer section from the inner section and each electrical heater is comprised of a film electrical resistance disposed along a longitudinal dimension of the outer section of the seal forming element, the electrical resistance being distributed along the longitudinal dimension with a higher electrical resistance being adjacent to a part of the intermediate section through which heat flows to the inner section and a lower electrical resistance which is disposed adjacent to the thermal insulation of the intermediate section in accordance with the invention includes applying an electrical potential to each electrical resistance to heat the seal forming elements at a point of moving contact with the at least one web to form the thermal seals on the moving at least one web. The inner section of each electrical heater may be joined to upper and lower parts of the wheel which are separated along an axis of rotation of the wheel and the higher electrical resistance may be located proximate to the upper and lower parts of the wheel and the lower electrical resistance may separate the higher electrical resistance proximate to the upper and lower parts; and during rotation of the sealing wheel to form the thermal seals a variation in temperature along a longitudinal dimension of the electrical heater during rotation may be less than 30xc2x0 F. The variation in temperature may be preferably less than 15xc2x0 F. and most preferably may be less than 5xc2x0 F. During rotation of the wheel to form the thermal seals, rotation of the wheel and movement of the at least one web may be stopped while the at least one web contacts the periphery of the wheel, the electrical potential may be disconnected from the electrical resistance to stop heating and seals are produced which are perpendicular to a longitudinal dimension of the web which are straight and parallel.
A sealing system which forms spaced apart thermal seals along a moving at least one web by contact of the moving web with heated portions of sealing elements mounted on a sealing wheel during rotation in accordance with the invention includes seal forming elements which peripherally contact the at least one web to form the thermal seals by heat transfer; and wherein each seal forming element has an outer section on which is located at least one electrical heater, an inner section which contacts the sealing wheel and an intermediate section which includes thermal insulation which insulates the outer section from the inner section, and the intermediate section has two end projections which separate the outer section from the inner section, each electrical heater is comprised of a film electrical resistance disposed along a longitudinal dimension of the outer section of the seal forming element, the electrical resistance being distributed in a plurality of sections of different electrical resistance along the longitudinal dimension and a higher electrical resistance section is disposed at ends of the outer section and at least one lower electrical resistance section is disposed from the higher resistance sections to a midpoint between the ends. The film electrical resistance may be a thick film electrical resistance. The at least one lower electrical resistance section may include a plurality of lower electrical resistance sections which are disposed symmetrically about the midpoint. The plurality of lower electrical resistance sections may comprise a lowest electrical resistance section disposed symmetrically about the midpoint and pair of electrical resistance sections may be disposed between the higher electrical resistance sections and the lowest electrical resistance section. At least a pair of film resistances may be disposed on opposed surfaces of each outer section and a seal forming surface may touch the moving at least one web, may be opposed to the thermal insulation and may intersect the opposed surface of the outer section. A single film resistance, which preferably is a thick film resistance, may be located in the seal forming surface which contacts the at least one moving web.
A thermal sealing element in accordance with the invention includes a support; a film heating element having a resistance to which is applied electric current for generating heat to thermally seal film contacting the heating element; and insulation separating the film heating element from the support. The support may be a mounting member which attaches to a movable sealing system. The film resistance may have a thick film resistance; and the thick film resistance may have a substrate which is a metallic member which is joined to the insulation. The insulation may be a solid insulation layer which separates the film heating element from the support. The insulation may comprise a section having a slot providing an air gap between the film heating element and the support with the section having a pair of projections which separate the support from the film heating element and the support and which define ends of the air gap. The film resistance may vary along a length thereof. The film resistance may be comprised of a plurality of sections with at least two of the sections having a different resistance.
A sealing system which forms spaced apart thermal seals along at least one moving web by contact of the moving at least one web with heated portions of an outside periphery of a sealing wheel during rotation in accordance with the invention includes seal forming elements attached to the sealing wheel which peripherally contact the at least one web to form the thermal seals by heat transfer to the at least one web; and wherein each seal forming element has an outer section which peripherally contacts the at least one web and includes a film resistance electrical heater facing the at least one web and thermal insulation disposed between the outer section and the sealing wheel which insulates the outer section from the sealing wheel. The seal forming elements may comprise a first group which we parallel to an axis of rotation of the sealing wheel, and a second group which are disposed circumferentially on an outside periphery of the sealing wheel and are individually disposed between different pairs of adjacent seal forming elements of the first group. The first and second groups may be continuous and joined together. The film resistance may be a thick film resistance which varies along a longitudinal dimension thereof. The thick film resistance of each sealing element may be comprised of a plurality of sections of resistance with at least two of the sections being of a different resistance. The film resistance may be a thick film resistance which varies along a longitudinal dimension of at least one of the first and second groups. The thick film resistance may be comprised of a plurality of sections of resistance with at least two of the sections being of a different resistance in at least one of the first and second groups. The first group may include at least one section of resistance disposed between ends thereof which is wider than the resistance at the ends which produces a seal dividing a container defined between adjacent elements of the first group and an element of the second group into at least two sections with the at least one section of resistance which is wider between adjacent elements producing a seal defining an opening into one of the at least two sections.
A sealing system which forms spaced apart thermal seals between at least two webs by contact of the at least two webs with heated seal forming elements in accordance with the invention includes a pair of seal forming elements which contact opposed sides of the at least two webs to form the thermal seals by heat transfer to the at least one web; and wherein each of the pair of seal forming elements has a first section which contacts one of the at least two webs and includes a film resistance electrical heater facing one of the at least two webs and thermal insulation disposed between the first section and a second section which is spaced farther from the at least two webs than the first section with the seal forming elements having a closed periphery for containing a substance inside the closed periphery between the at least two webs. The resistance may be a thick film resistance and the insulation may be a plurality of slots located at different sides of each seal forming element with each slot providing an air gap between the film heating element and the second section. The resistance may be a thick film resistance and the insulation is a peripheral ring of thermal insulation extending around a periphery of each seal forming element.